High-frequency oscillatory

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1 High-frequency oscillatory ventilation in adults: Respiratory therapy issues Jason Higgins, BS, RRT; Bob Estetter, RRT; Dean Holland, RRT; Brian Smith, RRT; Stephen Derdak, DO Objective: To summarize clinical information and assessment techniques relevant to respiratory therapists caring for adult patients on high-frequency oscillatory ventilation (HFOV). Data Source: Review of observational studies, controlled trials, case reports, institutional experience, and hospital HFOV guidelines for adult patients. Data Summary: Respiratory therapists require unique physical assessment skills and knowledge in managing patients on HFOV. Respiratory therapy procedures relevant to HFOV include setting endotracheal tube cuff leaks, performing lung recruiting maneuvers, endotracheal suctioning, and monitoring ventilator parameters. Respiratory therapists serve as essential team members in the creation and implementation of written HFOV guidelines (e.g., algorithms) to optimize patient care. Conclusion: Respiratory therapy assessment and procedural skills are essential in providing optimal care to adult patients on HFOV. (Crit Care Med 2005; 33[Suppl.]:S196 S203) KEY WORDS: high-frequency oscillatory ventilation; respiratory therapy; acute respiratory distress syndrome; lung recruitment maneuvers; endotracheal cuff leak High-frequency oscillatory ventilation (HFOV) is an alternative mode of ventilation that may be considered for acute respiratory distress syndrome (ARDS) in adult patients who are failing conventional ventilation (CV). Observational studies have demonstrated that HFOV may improve oxygenation when used as a rescue modality in adult patients with severe ARDS failing CV (1, 2). A recent multicentered, randomized, controlled trial of HFOV (3), compared with pressure control ventilation, demonstrated that HFOV was safe and effective for adult ARDS. Of particular importance to the respiratory therapist (RT), HFOV does not offer traditional monitoring capabilities (e.g., tidal volume, flowtime graphs, flow-volume loops, and so on) used to identify and optimize conventional ventilator strategies for changes in pulmonary mechanics. A multidisciplinary approach is required to optimize the management of patients on HFOV. RTs caring for adult patients with ARDS should develop increased knowledge of mechanical properties intrinsic to HFOV, From Parkland Health and Hospital System (JH, BE, DH, BS), Respiratory Care Department, Dallas, TX; and Pulmonary/Critical Care Medicine (SD), Wilford Hall Medical Center, Lackland AFB, TX. Copyright 2005 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: /01.CCM D an understanding of the underlying pathophysiology, and advanced patient assessment skills unique to this mode of ventilation. Clinical expertise coupled with advanced patient assessment skills place RTs in a key position in the management of patients on HFOV. The purpose of this chapter is to summarize clinical information pertinent to RTs caring for adult patients on HFOV. Active involvement of a critical care respiratory therapy team is essential to successful implementation of an adult HFOV program. IDENTIFYING PATIENTS FOR HIGH-FREQUENCY OSCILLATORY VENTILATION Observational rescue trials suggest that early initiation of HFOV in patients with severe ARDS may be important to successful outcomes. Patients transitioned to HFOV within 72 hrs may have a better chance of survival than those patients on CV for 7 days (1, 2). RTs, as an integral member of the critical care team, are in a frontline position to actively identify patients with ARDS who may be potential candidates for a trial of HFOV. Although the exact severity threshold at which to initiate a trial of HFOV remains unclear, an emerging approach in centers experienced in treating adults with HFOV may serve as guidelines (4, 5). HFOV may be considered for patients with ARDS when they meet the following severity criteria: FIO 2.60 and/or SpO 2 88% on CV with positive end-expiratory pressure 15 cm H 2 O, or Plateau pressures (Pplat) 30 cm H 2 O, or Mean airway pressure (MAP) 24 cm H 2 O, or Airway pressure release ventilation P high 35 cm H 2 O. Once severity criteria are met, and a decision is made to initiate HFOV, emphasis should be placed on transitioning to HFOV as soon as feasible (within hrs). HIGH-FREQUENCY OSCILLATORY VENTILATION TEAM APPROACH At Parkland Hospital, the HFOV management team consists of the attending intensivist, respiratory care team leader, respiratory care area manager, intensive care unit respiratory therapist, critical care nurse, and an identified consult team member who is on-call 24 hr a day/7 days a wk for troubleshooting (Table 1). Once a potential candidate is identified, the HFOV team will discuss possible reasons the patient is failing CV, review other available options (e.g., prone positioning), timing of transition from CV to HFOV (e.g., will bronchoscopy be per- S196

2 Table 1. High-frequency oscillatory ventilation (HFOV) consultation team Responsibility Intensive care unit respiratory therapist Respiratory care team leader Consultation team members RC, respiratory care; TL, team leader. formed before initiation?), initial HFOV settings, baseline monitoring measurements, and HFOV strategies for a failureto-respond scenario. An important goal of the HFOV team approach is to facilitate communication between disciplines and to ensure that parameter adjustments are appropriate. The intensive care unit RT is responsible for identification and hands-on care of the patient after initiation. He or she is the liaison to the HFOV team and has been trained and demonstrated to be competent in caring for patients on HFOV. We believe the team approach provides our patients with the best care by optimizing HFOV management and providing ongoing education for those individuals not as familiar with the management of HFOV. Patient Assessment and Monitoring Techniques Action/circumstance Identify patients that meet the indications for implementation of HFOV Contact RC team leader when patient is identified Record current settings on conventional ventilation before implementation of HFOV Contact RC team leader when ventilator changes are indicated Consult with TL, consultation team members, and physician staff regarding all setting changes Assure that HFOV is available and properly calibrated Play an active role in the management of the HFOV Contact identified consultation team member when ventilator changes are indicated Consult with intensive care unit therapists, consultation team members, and physician staff regarding all setting changes Play an active role in the management of the HFOV Consult with team leaders, therapists, other team members, and physician staff regarding all setting changes Respond promptly when on-call for consultation Provide pertinent articles, troubleshooting guides, and other material to interested therapists and physicians RTs must have a broad understanding of ARDS pathophysiology, mechanical properties of the mode of ventilation being used, and the specific ventilator strategy and goals chosen for the patient. A thorough physical assessment is, undoubtedly, the most important skill needed to provide quality care to patients on HFOV. RTs receive extensive training in auscultation, inspection, palpation, and percussion (6). In addition, they receive thorough training on how to interpret the monitoring indices associated with CV, as well as how to troubleshoot a ventilator (7). At present, literature regarding physical assessment and specific monitoring parameters for adult patients on HFOV is limited. Patients on HFOV may experience acute changes requiring rapid recognition to provide optimal management. Identifying Diaphragm Position. Lung inflation during HFOV may be estimated by monitoring diaphragm position on supine portable chest radiographs (CXR). Hyperinflation may be suspected on CXR if the apical to diaphragm distance exceeds cm and/or the anterior sixth rib is visible above the diaphragm (8). Because changes in lung inflation may be accompanied by positional changes of the diaphragm, it is beneficial to identify baseline diaphragm position at the initiation of HFOV. The traditional technique of percussion to locate diaphragm position is difficult as a result of the noise generated by the ventilator. To accurately percuss the diaphragm, ventilation may need to be temporarily interrupted by stopping piston oscillation. At Parkland Hospital, we have used a Doppler technique to quickly identify the position of the diaphragm at the bedside. After initiation of HFOV, and simultaneously with the initial CXR, a pencil Doppler (9.3 mhz, model 915BL; Parks Medical Electronics, Aloha, OR) is used to locate the diaphragm. Diaphragm position is then marked directly on the patient with a surgical marker pen. This mark is used to correlate the position of the diaphragm with its location on the CXR. Once the location of the diaphragm has been confirmed, we then monitor lung displacement as part of routine patient assessment with every ventilator check. If trended consistently, this technique assists with early recognition of changes in lung inflation and may prompt ordering of a CXR if a significant change is suspected. Other techniques that may be used at the bedside to locate the position of the diaphragm are diaphragmatic auscultation or conventional ultrasound visualization. Auscultation. Breath sounds should be auscultated with every ventilator check and during patient compromise. Recommendations for frequency of HFOV ventilator checks by the RT are every 30 mins during the first hour after initiation, then every hour for 2 hrs, and then every 2 hrs (Table 2). Although bilateral auscultation of the chest may not reveal adventitious breath sounds, it can assist in the identification of lung inflation, diaphragm position, and may be helpful with detection of other complications such as pneumothorax, atelectasis, endotracheal tube (ETT) obstruction, or mucus plugging. Unlike the assessment of breath sounds during CV, the clinician may not be able to identify wheezes, rhonchi, or crackles during HFOV because of the small tidal volumes delivered and the noise of the oscillator piston. For this reason, breath sounds should be auscultated whenever HFOV is interrupted for manual ventilation. During HFOV, the clinician should listen closely to the quality of the percussions delivered by the ventilator. Breath sounds should be auscultated over all accessible regions of the chest and compared with the opposite side for symmetry. Unilateral decreased breath sounds may be detected with pneumothorax, mucous plugging, atelectasis, mainstem intubation, and pleural effusions. Bilaterally decreased breath sounds may be observed with accidental extubation, ETT occlusion (partial or complete), alveolar collapse, alveolar overdistension, fluid overload/pulmonary edema, and obesity. Visual and Tactile Inspection. It is very important to visualize and palpate the chest with each HFOV assessment. Visual inspection of the chest for chest wiggle and movement of the abdomen may assist with identifying changes in S197

3 Table 2. High-frequency oscillatory ventilator (HFOV) check sheet HFOV checks to be done every 30 mins 2, every 1 hr 2, then every 2 hrs or as ordered Date and time 3 (30 Mins) (60 Mins) (2 Hrs) (3 Hrs) (5 Hrs) (7 Hrs) (9 Hrs) FIO 2 mpaw Power P Hz % I-time System temperature Bias flow Cuff leak SpO 2 Heart rate Blood pressure OI Diaphragm monitored Bilateral BS Spon resp rate s in mpaw s in P 1 mpaw alarm 2 mpaw alarm Pressure limit Consult Team Notification Parameters SpO 2 drop of 3 5% without recovery mpaw drift 5cmH 2 O (with spontaneous breathing) May need increase in paralytic or sedation P drift 5 cmh 2 O with significant diaphragm change or P drift 10 cm H 2 O without significant diaphragm change ABG parameters ph 7.45 or 7.20 or a of 10 PaCO 2 60 mm Hg or 30 mm Hg or a of 10 mm Hg PaO 2 90 mm Hg or 55 mm Hg or a of 10 mm Hg Changes made 3 lung compliance, lung resistance, and/or airway resistance. If the chest is not bouncing as much as it was 2 hrs previously and there appears to be slight changes in ventilator parameters, there may be significant changes occurring in lung mechanics. Serial evaluations of rib spaces may also assist in determining lung inflation. Rib spaces that have increased over time may indicate the lung has been recruited to the point of overdistension. Obesity or chest wall edema may make these monitoring techniques difficult. ENDOTRACHEAL TUBES Proper ETT management is of extreme importance in all critically ill patients, especially those receiving HFOV. ETT size, position, and patency have direct effects on gas exchange independent of alterations in the patients underlying lung pathology. Smaller-diameter ETTs (e.g., 7 mm internal diameter) attenuate delivered tidal volume and make effective ventilation of large adults more difficult. Endotracheal Tube Position ETT position should be checked regularly and maintained. As a result of high levels of mean airway pressure (mpaw) during HFOV, migration of the ETT proximally in the trachea may occur. The position of the ETT relative to a fixed anatomic site (e.g., upper front teeth or gum) should be recorded and monitored frequently. Migration of the ETT as little as 2 to 3 cm can adversely affect the ability to ventilate the patient. Tracheal Suctioning Gross pulmonary edema, hemorrhage, or foaming into the airway, ETT, and/or oscillator circuit will impede the ability to oxygenate and ventilate during HFOV. Obvious filling of the ETT tube with edema, blood, or foam must be cleared by tracheal suction (usually combined with vigorous manual ventilation with an attached positive end-expiratory pressure valve) before initiation of HFOV. Similarly, excessive secretions or mucus plugging in the distal airways or ETT can adversely affect adequate gas exchange during HFOV. Because the mechanism of injury associated with ARDS predisposes to alveolar collapse, the RT must be aware that tracheal suctioning (TS) can also be detrimental by creating negative carinal pressure, which promotes additional alveolar derecruitment. For this reason, TS should be performed only when clinically indicated (e.g., visible secretions), especially in patients with marginal oxygenation requiring high mean airway pressures (mpaw). To ensure ETT patency, the inline TS catheter can be passed (without turning on suction) every 2 to 4 hrs along with instillation of a small volume of sterile saline (2 3 ml). We perform tracheal suction on CV just before initiation of HFOV and then briefly clamp the ETT during transition to minimize alveolar derecruitment. It should be noted that this clamping technique is used in any instances the patient requires a ventilator disconnect (e.g., bronchoscopy, TS, transporting). If possible, TS is avoided during the first 12 hrs on HFOV to allow alveolar recruitment. After this S198

4 initial timeframe, in-line, closed TS is performed only when necessary and not on a routine scheduled interval. Additional indications for TS include an abrupt increase in proximal oscillatory amplitude ( P) coupled with decreased chest wiggle, unexplained hypercapnia or increasing oxygen requirements, and following prone positioning. If the patient s respiratory status continues to deteriorate in the presence of excessive pulmonary secretions after TS, bronchoscopy may be considered. Patients who desaturate (SpO 2 drops 5%) after TS should be considered for performance of a lung recruiting maneuver (LRM; see subsequently). Endotracheal Tube Obstruction ETT narrowing caused by inspissated mucus or blood clot accumulation will result in increased airway resistance secondary to a decreased lumen size. This increase in airway resistance can result in increased proximal oscillatory pressure amplitude ( P proximal ) displayed on the ventilator and decreased carinal oscillatory pressure amplitude ( P carinal ). Partial occlusion of the ETT should be suspected if a previously stable PaCO 2 is now increasing. Identification may also be reflected by an acute or gradual increase in P proximal ( 5 cmh 2 O) coupled with a decrease in chest wiggle, a decrease in breath sounds bilaterally, and an increasing oxygen requirement. Acute occlusion of the ETT from mucous plugging or a kinked tube presents with a sudden increase in P proximal ( 10 cm H 2 O), decreased chest wiggle, decreased breath sounds bilaterally, and rapid oxygen desaturation with hypercapnia. It is imperative the RT understands that decreasing the power setting in an attempt to maintain an ordered P proximal will only mask the underlying problem. Moreover, decreasing the power may result in further reduction of P carinal, which may compromise the patient more, even though the proximal amplitude appears to be unchanged. The possibility of complete ETT obstruction should initially be assessed by passage of a suction catheter and can be definitively diagnosed with emergent fiberoptic bronchoscopy (FOB). If a suction catheter cannot be passed and manual ventilation produces no air movement, emergent reintubation is required. An attempt may be made to pass an ETT exchange catheter or stylet in an effort to open the occlusion while equipment is readied to reintubate. Quicklook bronchoscopy should be considered before initiation of HFOV to ensure ETT and airway, especially if on CV 3 days. Patients who desaturate significantly after bronchoscopy may benefit from LRM (see subsequently) before resumption of the desired mpaw setting. Cuff Leaks Cuff leaks during HFOV may promote PaCO 2 clearance by several mechanisms and may allow for the use of lower P and higher Hz (which are conceptually more lung protective) (9 11). A small cuff leak, approximately 5 7 cm H 2 O, may be tried when refractory hypercapnia (ph 7.20) occurs despite maximal P and lowest Hz. Failure of a cuff leak to lower PaCO 2 may indicate upper airway edema around the ETT and may respond to placement of an additional oropharyngeal airway to allow gas egress (12). Some centers use ETT cuff leaks at the initiation of HFOV in all patients. Whether to reserve use of a deliberate cuff leak for refractory hypercapnia or to use a leak in all patients to facilitate use of lower P and higher Hz strategies should be investigated. Before creating a cuff leak, the mouth and posterior pharynx should be suctioned. The low mpaw and high mpaw alarm should be reset to avoid triggering by a drop or rise in mpaw with initial setting of the cuff leak. At Parkland Hospital, our approach is to initiate a cuff leak by increase bias flow by 5 L/min, then slowly remove air from the ETT cuff pilot balloon while monitoring for a 5- to 7-cm H 2 O drop in mpaw on the ventilator. Once the appropriate leak has been applied, the mpaw control is readjusted to return the mpaw to the original setting. It should be noted that increasing bias flow after institution of a cuff leak to achieve a set mpaw may result in an elevated mpaw as a result of a decreased cuff leak. This is of clinical importance because the magnitude of the cuff leak may change as a result of tracheal edema, secretion accumulation, and body positioning. The mpaw alarms and mpaw pressure limit should be set appropriately (e.g., bracket desired mpaw by 5 7 cm H 2 O) to protect the patient in the event of a decreasing cuff leak. The technique used to create a cuff leak at Wilford Hall Medical Center is detailed in Appendix 1. HUMDIFICATION Humidification is often overlooked as an important aspect during any form of mechanical ventilation. Because endotracheal intubation bypasses the upper airway, it becomes necessary for inspired gases to be heated and humidified artificially to mimic normal respiratory physiology (13). Complications that may occur as a result of ineffective heat and humidification are, but not limited to, hypothermia, inspissation of airway secretions, destruction of airway epithelium, and atelectasis (14). There are currently two forms of delivering heat and humidification to patients requiring mechanical ventilation: external active humidifiers and passive heat and moisture exchangers (HME). HMEs have not been adequately studied with HFOV and should not be used. During HFOV, the bias flow circuit is connected directly to an external active humidifier to provide humidified gas entering the inspiratory limb of the circuit. Temperature settings should resemble those normally used during conventional mechanical ventilation and should be set to establish the desired gas temperature at the patient airway temperature port (15). We suggest maintaining temperature settings at 37 C to 39 C. Water levels in the chamber should always be maintained at the appropriate levels to prevent the chamber from becoming dry. This will result in the patient receiving only heated gas without proper humidification and may result in complications. The HFOV circuit has two temperature ports on the inspiratory limb, one near the patient s airway and one near the pressure limit valve. Temperature should always be monitored as close to the patient s airway opening as possible. Because ambient air temperatures can affect the temperature and relative humidity in the circuit, caution should be exercised if ambient temperatures exceed 84 F (e.g., burn intensive care units). Also, to prevent excessive rainout in the circuit, a heated wire circuit should be used. At our institution, active external humidifiers are checked with each ventilator check and appropriate documentation is performed. LUNG RECRUITMENT MANEUVERS Lung recruitment maneuvers are used to improve oxygenation after derecruit- S199

5 ing events (e.g., suction, bronchoscopy, circuit disconnects) or for patients who continue to have marginal oxygenation during HFOV. LRMs should be considered for an acute drop in SpO 2 5% after initial HFOV transition, TS, bronchoscopy, or circuit disconnect. When using an in-line closed suction catheter, performance of a recruitment maneuver during TS has been shown to prevent alveolar derecruitment during CV (16). Lung recruiting maneuvers are typically performed by briefly (40 60 secs) raising mpaw approximately 10 cm H 2 O above the original set mpaw. Before performing the maneuver, the mpaw high alarm must be reset (e.g., to 50 cm H 2 O) and any ETT cuff leak removed. The oscillator piston is turned off during the maneuver to minimize additional distal transmission of the P while the mpaw is elevated. Suggested LRM guidelines for HFOV are detailed in Appendix 1. PRONE VENTILATION Prone positioning is becoming a more widely used therapeutic modality in patients with refractory hypoxemia. Studies show an improvement in oxygenation in approximately 60% of patients with ARDS (17, 18). Case reports have observed improvements in oxygenation and ventilation with the combination of HFOV and prone ventilation (19). When patients receiving HFOV are placed in the prone position, patient and ventilator assessment become extremely important. Careful adherence to a detailed proning algorithm is essential (20). Like with all patients receiving mechanical ventilation, maintaining a patent airway is of extreme importance. The actual turning of the patient usually requires brief disconnect from the ventilator circuit and manual ventilation with a positive endexpiratory pressure valve. ETT placement must be confirmed and the circuit should be resecured after prone positioning. HFOV parameters need to be verified and documented immediately before and immediately after prone positioning. Lung mechanics and ETT leaks can rapidly change with patient turning, and RTs must be prepared to recognize these changes and address them rapidly. LRMs may be performed in the prone position, particularly if a circuit disconnect was required and desaturation persists. TENSION PNEUMOTHORAX Like with all forms of positive pressure ventilation, tension pneumothorax may develop as a manifestation of volutrauma or secondary to the cystic nature of the underlying lung disease. Additionally, vigilance must be maintained for pneumothorax in patients undergoing central line placement (e.g., subclavian or internal jugular) and thoracentesis. Studies have demonstrated pneumothorax occurrence rates during HFOV similar to those observed with conventional ventilation. However, in contrast to CV, the presence of a pneumothorax can be particularly difficult to detect in patients on HFOV because no alarms on the ventilator will reliably signal that tension is developing (21). Quick assessment of ETT placement, hemodynamic parameters, tracheal position, visualization of the chest for unilateral hyperinflation, decreased chest movement, auscultation for breath sounds, diaphragmatic position, and palpation to identify the presence of subcutaneous emphysema assist the RT in detection of pneumothorax. If time permits and the patient is relatively stable, the diagnosis can be confirmed by a stat portable CXR. After placement of a chest tube, the RT should anticipate that adjustments in mpaw and P:Hz will be required. The degree of leak (e.g., from a bronchopleural fistula) should be quantitated on the chest tube suction chamber device, and changes in the leak should be recorded in response to changing HFOV settings and as part of the routine scheduled ventilator checks. Air leak through a bronchopleural fistula can be minimized by using the highest Hz, the lowest P, the lowest mpaw, and the shortest inspiratory time (IT%) allowable to achieve acceptable oxygenation and ventilation (22). HEMODYNAMICS, ARTERIAL PRESSURE TRACINGS, AND PULSE OXIMETRY Central vein and pulmonary artery pressure monitoring, peripheral arterial catheters, and pulse oximetry are forms of real-time monitoring that assess patient hemodynamic stability during HFOV. Hemodynamic status is extremely important before and after initiation of HFOV. HFOV maintains alveolar recruitment by sustaining an essentially constant mpaw (in contrast to the cyclic pressure excursions of conventional volume-cycled ventilation). This increases mean intrathoracic pressure, reduces central venous return (e.g., right heart preload), and may cause potential adverse effects on the cardiovascular system. Therefore, patients requiring HFOV should be hemodynamically stabilized before initiation and a fluid bolus and/or vasopressors should be readily available if hypotension occurs. We commonly use central venous pressure monitoring (e.g., internal jugular or subclavian central vein) or pulmonary artery flotation catheter monitoring to ensure optimal hemodynamics in unstable patients. In the absence of a pulmonary artery flotation catheter, heart rate and blood pressure can be useful. Dampened arterial and pulse oximetry waveforms may indicate compromised cardiac output and require special attention by the RT to trend (7). In hypovolemic patients, heart rate readings on pulse oximetry may be observed that reflect the oscillatory frequency being delivered. This phenomenon may suggest that cardiac output is being compromised as a result of high intrathoracic pressures. In addition, waveform tracings of central venous pressure or pulmonary capillary wedge pressure (PCWP) may sometimes show low-amplitude superimposed waves that correspond to the HFOV frequency (Hz). Brief interruption of the oscillator piston (while maintaining mpaw) may be considered when trying to obtain an accurate assessment of central venous pressure or PCWP waveforms. Changes in mpaw may produce similar changes in central venous pressure or PCWP, suggesting that at least some of the mpaw is being transmitted to the intravascular pressures. The response of blood pressure, central venous pressure, or PCWP trends in response to fluid challenges may be a better indicator of the patient s intravascular volume status than any absolute value, particularly in hypotensive patients requiring high mpaw. OPTIMIZING HIGH-FREQUENCY OSCILLATORY VENTILATION SETTINGS To optimally manage critically ill patients on HFOV, it is important to understand the machine s capabilities and limitations. HFOV has been considered a decoupling device. By definition, a decoupling device uses individual controls that affect only certain parameters and nothing else. For example, mpaw, FIO 2, S200

6 and inspiratory time % (IT%) primarily affect oxygenation, whereas increasing oscillatory pressure amplitude ( P), decreasing frequency (Hz), and creating ETT cuff leaks primarily increase ventilation. Clinical observations, however, suggest that HFOV is not an absolute decoupling device. To maximize gas exchange while minimizing ventilator-associated lung injury, it is imperative to maintain an optimal mpaw while avoiding excess tidal volume delivery. Alveolar overdistension or underdistension from an improperly set mpaw or P:Hz combination can adversely affect both oxygenation and ventilation respectively and potentiate further lung injury. Optimizing Mean Airway Pressure Determining an appropriate mpaw setting can prove to be particularly challenging, especially during recruitment phases. Optimal mpaw can be described as that mpaw which causes sufficient lung inflation to maximize gas exchange while protecting the lung from alveolar overdistension, underdistension, or impeding hemodynamics. In hemodynamically stable adults, mpaw is typically started at 2 5 cm H 2 O above the mpaw observed during CV. Subsequent increases in mpaw by 1 2 cm H 2 O every mins are used (up to a maximum of40 45cmH 2 O) to achieve a target SpO 2 88% with an FIO 2 60%. Whether to wean mpaw before reducing FIO 2 in patients who require high mpaw (e.g., 35 cm H 2 O) for optimal lung protection remains unclear. One approach to using combinations of mpaw, FIO 2, and lung recruiting maneuvers for oxygenation and weaning is outlined in Appendix 1. Application of technologies such as respiratory inductance plethysmography and electrical impedance tomography during HFOV may offer more precise bedside tools for assessing lung inflation and are reviewed elsewhere in this supplement. OSCILLATORY PRESSURE AMPLITUDE ( P) RANDOM DRIFT OR PATIENT FEEDBACK? During HFOV, real-time feedback and trending of airway and lung mechanics may be available through monitoring changes in the displayed P (21, 23). Understanding that a number of clinical variables may cause changes in P can provide the bedside clinician with a useful monitoring tool. Increases in P (assuming constant power setting) may occur with increases in ETT resistance, bronchospasm, or mainstem intubation (21). Thoracic compliance changes may have variable effects on P and may not be distinguishable from airway resistance effects. Similarly, variable ETT cuff leaks or spontaneous breathing may cause fluctuations in P. As a result of the many contributing factors associated with changes in P, patient care decisions should not be made solely on this measurement. Rather, P should be included as an additional monitoring parameter in conjunction with other patient assessment techniques, hemodynamic parameters, and arterial blood gas analysis. We routinely record the observed P during ventilator checks and monitor trends as an early indicator of possible changes in pulmonary mechanics. In situations of acute decompensation, an understanding of the variables affecting P may give the clinician an additional tool to determine whether the cause of decompensation is pulmonary or nonpulmonary. Table 3 depicts various clinical situations and their possible effects on P. HIGH-FREQUENCY OSCILLATORY VENTILATION AND AEROSOL MEDICATION DELIVERY Very few studies have examined methods to optimize aerosolized medication delivery during HFOV. Metered dose inhalers (MDI) appear relatively ineffective in delivering optimal drug amounts. MDI delivery during HFOV has been shown to deliver approximately 1% to 2% of the aerosolized drug in a neonatal lung model and 2.5% to 6.3% in a pediatric lung model (24). In this study, the low deposition was attributed to turbulent flow in conjunction with the high bias flow in the system and the small diameter of the ETT. No difference was noted between differing settings on the HFOV. Use of flow-driven nebulizers in-line during HFOV has not been thoroughly studied. This form of aerosol generator provides an additional amount of flow to the circuit that will result in alterations of the mpaw and the P. During delivery of aerosolized medication, the RT must reduce oscillator bias flow to maintain constant mpaw (25). New aerosol generators that use a vibrational element to generate a low- Table 3. Affects of clinical situations on proximal amplitude ( P proximal ) and possible treatments Condition Compliance V t P carinal P proximal Treatment Alveolar overdistension Decreased Decreased Increased Decreased Decrease mpaw incrementally Tension pneumothorax Decreased Decreased Increased Decreased Decrease mpaw for adverse reactions Chest tube placement Mucous plugging Decreased or same Decreased Increased Decreased Bag, saline lavage and suction Bronchoscopy Bronchoconstriction Decreased Decreased Increased Decreased Bronchodilator Steroids Fulminating pulmonary edema Decreased Decreased Decreased Increased a Increase mpaw (endotracheal tube frothing) Increasing endotracheal tube resistance (partial occlusion) No Change Decreased Decreased Increased Bag, saline lavage, and suction Bronchoscopy Endotracheal tube occlusion No Change Decreased Decreased Increased a Bag, saline lavage, and suction Reintubation Alveolar recruitment Increased Increased Decreased Increased Monitor for over-distension a Situation may result in P proximal changes 10 cm H 2 O; P proximal is measured in the oscillator circuit and displayed on the ventilator. 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7 velocity, small droplet medication have showed promising results in optimizing drug delivery. This form of aerosol generator delivers up to the three times the amount of drug without effecting ventilator parameters when compared with traditional flow-driven aerosol generators (26). Aerosol drug delivery during HFOV should be investigated further to determine optimal techniques. The use of aerosolized selective pulmonary vasodilator medications is reviewed elsewhere in this supplement. CONCLUSION Integration of respiratory therapy expertise into the management of adult patients on HFOV is an essential component to successful outcomes with this novel mode of ventilation. Respiratory therapists serve as team leaders in the development and implementation of HFOV treatment algorithms. In addition, the RT plays a vital role in monitoring patients on HFOV, in early recognition of changing clinical conditions (e.g., obstructing ETTs, pneumothorax, hyperinflation), and in HFOV-related procedures (e.g., creating ETT cuff leaks, lung recruitment maneuvers). Optimal techniques to deliver aerosol medications during HFOV remain unclear and require further study. REFERENCES 1. Fort P, Farmer C, Westerman J, et al: Highfrequency oscillatory ventilation for adult respiratory distress syndrome a pilot study. Crit Care Med 1197; 25: Mehta S, Lapinsky SE, Hallet DC, et al: A prospective trial of high-frequency oscillation in adults with acute respiratory distress syndrome. Crit Care Med 2001; 29: Derdak S, Mehta S, Stewart TE, et al: High frequency oscillatory ventilation for acute respiratory distress syndrome: A randomized, controlled trial. 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Journal of Respiratory Care 2001; 46:1080A 26. Fink JB, Barraza P, Bisgaard J: Aerosol delivery during mechanical ventilation with highfrequency oscillation: an in vitro evaluation. Chest 2001; 120:277S The Appendix is on the next page. S202

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